1. Field of the Invention
This invention relates to the field of switching scheme for switched capacitor filters. In particular, the present invention can be applied to active switched capacitor bi-quad and/or leap frog filters.
2. Background Art
A filter is a circuit network that selectively passes certain ranges of frequencies. Typically, filters come in two types. Passive filters, which employ resistors and capacitors, and active filters, which employ active elements such as operational amplifiers. Filters are applied as low or high pass filters, to attenuate frequencies below or above a certain frequency. Additionally, a filter may act as a band pass filter, allowing passage of frequencies between two limits and attenuating frequencies outside these limits, or as a band split filter, passing frequencies outside two limits and attenuating frequencies within two limits.
One application of active filters is to solve integrodifferential equations such as arise from the use of sinusoidal forcing functions. Such an application utilizes filters, comprising operational amplifiers and switched capacitor networks, as integraters to find solutions for signals which can be described by first order differential equations. Such filters are known as first order filters. When the forcing function or input signal becomes more complex, second order filters are utilized. A second order filter filters signals that can be characterized by a linear differential equation including a second order derivative or by two simultaneous linear first order differential equations. The second order signals can be described by Laplace transforms and are resident in the frequency domain.
One particular type of second order filter is known as a bi-quad (for bi-quadratic) filter. A typical prior art switched capacitor (SC) bi-quad circuit includes two operational amplifiers. Each of these operational amplifiers act as first order filter poles so that the bi-quad circuit can be used to filter second order signals described by simulataneous first order differential equations. Associated with each operational amplifier are capacitors coupled through switches to a feedback loop. An example of such a prior art bi-quad filter is illustrated in FIG. 1 and is discussed in detail below. The ratios of the capacitors associated with the operational amplifiers determine the frequency response of the SC bi-quad circuit.
The SC bi-quad circuit, such as the one described above, is often used as a component in semiconductor chips. In the manufacture of semiconductor chips, it is desired to minimize the silicon area used in making the chip to maximize yield and reduce the cost per chip. Switched capacitors require relatively large amounts of silicon die area to fabricate. Therefore, if the area required for switched capacitors in a bi-quad filter could be reduced, the filter could be produced in a smaller area, leading to more efficient chip production.
Therefore, it is an object of the present invention to provide a method of switching to be used in filters for second order signals, which requires less silicon area than prior art filters. In particular, the present invention is directed toward a bi-quad filter using less silicon area than prior art bi-quad filters.